The construction diversion of the Tankeng Hydropower Station has the characteristics of big discharge and quick flow speed on its overflow cofferdam.The cutoff wall of cofferdam was constructed without river-close mouth.The construction measures of air closure first and then river closure are adopted to realize the quick excavation of deep foundation pit.The crest elevation of the main part of dam filling enhanced properly during flood season period and the protect measures that using excess diameter stones at where relative superiority of surface of dam.The surface of dam was less eroded after flowing over the cofferdam many times.All these measures provide experience for the construction of facing dam in the future.